The present invention relates generally to connecting rods for coupling crankshafts and pistons, and more particularly to the coupling of the large end of a one piece connecting rod to the crankshaft.
Connecting rods for coupling pistons and crankshafts are designed with a small ring-shaped end and a large ring-shaped end joined together by a rigid member that is connected to the outer periphery of each ring portion. The small ring-shaped end is designed as a bushing or as a press-fit connection for receiving a bearing for rotatably receiving a piston pin, and the large end is designed for rotationally receiving the crankpin journal.
Most simple connecting rods may be broadly categorized in two types. The first more complex and expensive type is a split ring connecting rod. In such connecting rods, the large end surrounding the crankshaft is formed as two C-shaped halves, each containing an inner bearing surface and either bolted together across the joint or split formed by the ends of the C-shaped section in position about the crankshaft. The second type is a one piece connecting rod in which the ring section forming a bearing around the crankpin is continuous, without bolted joints. The simplicity of the one piece connecting rod is offset by more demanding considerations in crankshaft design. One piece connecting rods require the crankshaft to be either assembled around the connecting rods or to be designed in such a way as to allow the engine to be assembled by passing the large circular shaped end of the connecting rods over an end of the crankshaft and along the crankshaft onto the crankpin. The present invention applies mainly to the on piece type of connecting rod used with crankshafts designed to allow the connecting rod to be assembled over crankpins.
Although the manufacture and assembly of this type of one piece connecting rod and crankshaft is inexpensive in comparison to the split ring connecting rod, there are problems of retaining the one piece rod in position on the crankshaft during engine operation. This is because, by design, the end of the connecting rod can slip on and off the crankshaft. In such assemblies, the rod may be guided in the direction of the crankshaft axis by a piston and/or wrist pin between the piston pin bosses on either side of the small end of the rod. Piston guided rods work acceptably as long as the forces tending to move the rod axially on the crank arm are small. In order to keep axial forces to a minimum, the rolling elements or rollers of the connecting rod bearings must remain aligned parallel to the crankpin axis.
In expensive engines, a bearing cage may be utilized to maintain the rollers in proper alignment. However, in less expensive engines in which bearing cages are not utilized, roller alignment is difficult to maintain because of tolerance variations. Once a roller or rollers skew and are no longer in alignment, axial forces in the rod may be generated which tend to move the large end of the rod axially on the crankpin. Such movement can cause binding of the bearing on the crankpin as the rod that is restrained at the top by the piston is cocked by axial loads on the large end of the rod. The large lever arm provided by the rod transmits rather large forces to the piston from the axial forces generated by the crankpin bearing. Thus, in addition to binding the crankpin bearing, roller misalignment or skewness can cause high piston loads and wear.
In some engines, the connecting rod is guided by shoulders on either side of the crankpin of the crankshaft. With crankshafts designed to permit the connecting rod to be passed over the end of the shaft until the connecting rod is in position, axial movement of the connecting rod may be restrained in one direction by a thrust flange integral with the crankshaft. However, no restraint is provided on the other side of the rod, resulting in a tendency for the rod to "walk" along the crankshaft. It is thereby desired to prevent excessive axial movement of the connecting rod along the crankshaft.